1. Field of the Invention
The present invention relates to a convergence measuring apparatus for measuring a color misconvergence amount on a screen of a color cathode-ray tube.
2. Description of the Prior Art
A convergence measuring apparatus is known in which a color misconvergence amount on a screen of a color cathode-ray tube in a color television receiver or color display device is automatically measured.
In a convergence measuring apparatus of this type, a dot or cross-hatched pattern is displayed on the screen of a color cathode-ray tube, and an image of the dot pattern or the like is picked up by an image pickup camera. The picked image is converted from analog data into digital data, which is stored in a memory. The digital data is then subjected to image processing, so that the centers of gravity of luminance distribution areas of red (R), green (G), and blue (B) pattern data are calculated. Shift amounts of the centers of the gravity of the respective color components are obtained to measure the degree of convergence.
For example, when a dot pattern is displayed on the screen of the color cathode-ray tube, the centers of gravity of the luminance distributions of the pattern are shifted from each other depending on dot pattern display positions. For example, when a red (R) dot pattern is displayed on the screen of the cathode-ray tube, a red (R) phosphor portion of the screen of the color cathode-ray tube emits light, while the green (G) and blue (B) phosphor portions do not emit light. For this reason, the luminance distribution of the dot pattern tends to be discontinuous. As shown in FIGS. 1a and 1c, the center of the dot pattern is located at the central position of symmetrical right and left red (R) phosphors, a center A of the gravity of the inherent luminance distribution coincides with a center B of the luminance distribution of the dot pattern, and no problem is posed. However, when a dot pattern display position is slightly shifted, as shown in FIG. 1b or 1c, the following problem is posed. That is, when a center B of the luminance distribution of the dot pattern is shifted as shown in FIG. 1b with respect to a center A of gravity of the inherent luminance distribution in, e.g., a 0.8 mm pitch stripe tube, the position is shifted by 0.075 mm in the negative direction (i.e., the left direction in FIG. 1b). Similarly, when the center B of the gravity of the luminance distribution of the dot pattern is shifted as shown in FIG. 1d with respect to the center A of the inherent luminance distribution, the center is shifted by 0.075 mm in the positive direction (i.e., the right direction in FIG. 1d). In this manner, the center of gravity of the luminance distribution of the dot pattern is shifted to a maximum of .+-.0.075 mm depending on the display position on the color cathode-ray tube. The same problem is posed in green (G) and blue (B) dot patterns. It is therefore difficult to obtain the accurate centers of gravity of the luminance distributions of the dot patterns. A convergence error occurs in a maximum range of .+-.0.15 mm, and convergence measurement cannot be accurately performed.
In order to solve the above problem, a specific convergence measurement tube such as a tube having no shadow mask or a monochrome tube may be used. In this case, the resultant apparatus results in a special-purpose apparatus which can measure convergence for specific models and loses versatility.